Conventional methods for adding water-insoluble photographic additives to photographic silver halide emulsions include methods wherein a solution of the waterinsoluble photographic additives in an organic solvent is added to water, or to an aqueous solution containing an anionic surfactant, or to an aqueous solution containing an aqueous binder, or to a hydrophillic colloid solution to thereby crystallize the water-insoluble additives. The crystallized additives are brought into easily dispersible crystal states and then dispersed. The resulting dispersion is added to a silver halide emulsion.
Methods of mechanical dispersion them are described, for example, in U.S. Pat. No. 3,788,857, JP-A-50-11419 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), U.S. Pat. No. 3,660,101 and JP-B-49-46416 (the term "JP-B" as used herein means an "examined Japanese patent publication").
A method for forming a dispersion of waterinsoluble additives in water by removing the organic solvent after recrystallization is described in JP-A-49-128725.
Further, JP-B-61-45217 discloses a method different from the above-described methods wherein substantially water-insoluble photographic additives are mechanically dispersed in water at a pH of 6 to 8 and at a temperature of 60.degree. to 80.degree. C. in the absence of any organic solvent or surfactant and the resulting dispersion is then added to a silver halide emulsion.
When the water insoluble photographic additives are dissolved in organic solvents, and particularly when additives which are also poorly soluble in organic solvents are used, agglomerates tend to form resulting in streaks or lumps in the emulsion coating since large amounts of the organic solvents are used to dissolve the additives.
In the method wherein the dispersion is formed in water by removing the organic solvents after recrystallization, the organic solvents are removed by evaporation or by a separation method employing a membrane. The resulting solution after evaporation causes variability in concentration and the resulting composition is decomposed. The solution obtained by separation using a membrane also causes variability in concentration. Further, the method manufacturing process is complicated. In the method disclosed in JP-B-61-45217, some additives are left behind as coarse crystals (5 to 30 .mu.) without having been dispersed. It has been found that some additives can not be mechanically dispersed unless their crystal states are altered by any method.
The present inventors have previously proposed a process for preparing a silver halide emulsion, which solves the above-described problems, does not form lumps or streaks in the coating of the emulsion, causes neither variability in the concentration of the solution nor results in the decomposition of the composition, and allows the water-insoluble photographic additives to be easily dispersed and added to the emulsion. Namely, the present inventors have previously proposed a process for preparing a silver halide emulsion, which comprises dissolving substantially water-insoluble photographic additives in a mixed solution consisting of an organic solvent, a surfactant having hydrophilic -SO.sub.3 or -OSO.sub.3 groups and optionally, a small amount of a base or small amounts of a base and an acid, by heating the resulting adding the solution dropwise into water to recrystallize the substantially water-insoluble photographic additives, dispersing the additives by highspeed agitation, and adding the resulting dispersion to a silver halide emulsion.
Using the above method, it has been found that lumps or the streaks tend to form in the emulsion coating, and that this problem is caused by the organic solvent contained in the dispersion which is added to the silver halide emulsion. Thus, it is highly desirable to provide a method wherein the substantially water-insoluble photographic additives are added to the emulsion in the absence and an organic solvent to eliminate the above problem.